Ink jet recording method

ABSTRACT

An ink jet recording method includes ejecting at least two color ink compositions onto a recording medium so as to be deposited one on another. When one ink composition is deposited to form a first image and then another ink composition is deposited to form a second image on the first image, the ink composition forming the second image has a higher yield value than the ink composition forming the first image.

BACKGROUND

1. Technical Field

The present invention relates to an ink jet recording method.

2. Related Art

The ink jet recording method is a method for printing performed byejecting droplets of an ink onto a recording medium, such as a papersheet, from an ink jet head. The ink jet recording method is beinginnovatively developed and increasingly applied to high-resolution imagerecording (printing), which has been performed by photo printing andoffset printing.

In one of the ink jet recording methods, a line head including inknozzles arranged in a line ejects droplets of an ink onto a recordingpaper being transported at a speed corresponding to the ejection speedand volume of ink droplets. For example, JP-A-2002-103638 discloses arecording method using an ink jet printer including a recording headhaving ink nozzles through which an ink is ejected. This recording headis a line head in which lines of ink nozzles are arranged for respectivecolor inks and extend in a direction perpendicular to the direction inwhich the recording medium is transported. The nozzle lines have spacesin the recording medium-transport direction between each color, and thenozzle lines for the respective color inks are arranged in the recordingmedium-transport direction in such a manner that the hue of the colorink at the upstream side of the transport direction is lighter than thatof the downstream side.

The present inventors however found that this ink jet recording methodcauses color bleeding when at least two color inks are deposited on arecording medium in such a manner that one color ink overlies another.

This color bleeding is a phenomenon in which adjacent different colorsbecome indistinct, and is caused by the different colors spreading intoeach other to be mixed at the boundary between the different colors.

SUMMARY

Accordingly, an advantage of some aspects of the invention is that itprovides an ink jet recording method that can prevent color bleedingwhen at least two color inks are deposited on a recording medium in sucha manner that one color ink overlies another.

As a result of intensive research for a solution of the above issue, thepresent inventors found that whether color bleeding occurs depends uponthe dynamic viscosity of an ink deposited on a recording medium or on animage formed immediately before the ink is deposited. The inventors haveaccomplished the invention through intensive research for an ink jetrecording method capable of preventing color bleeding.

According to an aspect of the invention, an ink jet recording method isprovided in which printing is performed by ejecting at least two colorink compositions onto a recording medium so as to be deposited one onanother. When one ink composition is deposited to form a first image andthen another ink composition is deposited to form a second image on thefirst image, the ink composition forming the second image has a higheryield value than the ink composition forming the first image.

In this method, the ink composition previously ejected to form the firstimage has a relatively high fluidity immediately after landing on arecording medium. However, the fluidity has been reduced when anotherink composition is deposited. Therefore, even if another ink compositionis deposited on the first image in such a state, color bleeding of theink composition of the first image into the ink composition of thesecond image can be suppressed because of the low fluidity of the inkcomposition of the first image. On the other hand, the ink compositionforming the second image originally has a high yield value, and itsfluidity is low even immediately after it is deposited. Therefore, colorbleeding of the ink composition of the second image into the inkcomposition of the first image can also be suppressed. Thus, the ink jetrecording method can prevent the occurrence of color bleeding.

If the relationship of the yield values is reversed, that is, if the inkcomposition forming the second image has a lower yield value than theink composition forming the first image, the fluidity of the first imageis relatively low at the time when the ink composition of the firstimage is deposited, and the low fluidity continues until another inkcomposition is deposed to form the second image on the first image.Therefore, when another ink composition is deposited on the first imagein such a state, color bleeding of the ink composition of the firstimage into the ink composition of the second image can be suppressedbecause of the low fluidity of the first image, as in the same mechanismas above. However, the ink composition forming the first imageoriginally has a lower yield value, and its fluidity is high accordinglyimmediately after it is deposited. Consequently, color bleeding canoccur from the ink composition of the first image to the ink compositionof the second image. In this instance, unlike the case of the ink jetrecording method of an embodiment of the invention, it is difficult toprevent the occurrence of color bleeding.

Preferably, the first image has a higher lightness than the secondimage. By ejecting ink compositions in decreasing order of lightness, animage formed particularly with an ink composition having a higherlightness can be prevented from being spread, in addition to theadvantage of preventing the occurrence of color bleeding caused bydepositing one ink composition on another. Accordingly, an image havinga still higher sharpness can be formed by the ink jet recording methodof an embodiment of the invention. For example, characters and linesformed with inks of cold colors, such as black and blue, which are oftenused in business application can be prevented from being spread into abackground image formed with inks of warm colors, such as red andyellow, which are also often used, and consequently, it seems that animage having a high sharpness has been formed.

In an ink jet recording method of an embodiment of the invention, thecolor ink compositions include a black ink composition having a yieldvalue Y_(K), a cyan ink composition having a yield value Y_(C), amagenta ink composition having a yield value Y_(M), and a yellow inkcomposition having a yield value Y_(Y). The yield values of these inkcompositions satisfy the relationship: Y_(K)>Y_(C)>Y_(M)>Y_(Y).

In an ink jet recording method of another embodiment of the invention,the color ink compositions include a black ink composition having ayield value Y_(K), a magenta ink composition having a yield value Y_(M),a cyan ink composition having a yield value Y_(C), and a yellow inkcomposition having a yield value Y_(Y). The yield values of these inkcompositions satisfy the relationship: Y_(K)>Y_(M)>Y_(C)>Y_(Y).

These embodiments of the ink jet recording method can prevent theoccurrence of color bleeding and can further form images having highsharpness.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a schematic view of an ink jet recording apparatus used in anembodiment of the invention.

FIG. 2 is a schematic perspective view of a part of an ink jet recordingapparatus used in an embodiment of the invention.

FIG. 3 is a schematic sectional view taken along line III-III shown inFIG. 2.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention will now be described in detail withreference to the drawings. However, the invention is not limited to theembodiments. Also, various modifications may be made without departingfrom the scope and spirit of the invention. In the drawings, the sameelements are designated by the same reference numerals and the samedescription will not be repeated. The relative positions and otherpositional relationship accord with the drawings unless otherwisespecified. The dimensional proportions in the drawings are not limitedto those shown in the drawings.

In an ink jet recording method according to an embodiment of theinvention, printing is performed by ejecting at least two color inkcompositions to form respective images on a recording medium so as to bedeposited one on another. When one ink composition is deposited to forma first image and then another ink composition is deposited to form asecond image on the first image, the ink composition forming the secondimage has a higher yield value than the ink composition forming thefirst image. The yield value and residual viscosity mentioned herein aredetermined as below. First, the shear rate of the ink composition at 20°C. is varied and the relationship between the shear rate and the shearstress is measured. Subsequently, the yield value and residual viscosityare calculated by applying the measured values to Casson Equation:√S=a×√D+b

In the Casson Equation, S represents the shear stress (unit: Pa), Drepresents the shear rate (unit: 1/s), and a and b each represent aconstant. Non-Newtonian fluid liquids, many of which apply to the Cassonequation, are used in considerably broad fields. The square of slope arepresents the residual viscosity, and the square of intercept brepresents the yield value. These are property values of a liquid. As isclear from the Casson equation, the residual viscosity refers to theviscosity at infinite shear rate, and the yield value refers to thestress at a shear rate of zero.

An ink jet recording apparatus used in the ink jet recording method ofthe present embodiment may have the structure shown in FIGS. 1, 2 and 3.FIG. 1 is a schematic view of the ink jet recording apparatus,particularly showing the paper transport portion, and FIG. 2 is aperspective view of a part of the recording apparatus. FIG. 3 is aschematic sectional view of the ink jet recording apparatus taken alongline III-III in FIG. 2, showing a state in which a recording medium,such as plain paper, is transported.

The plain paper mentioned herein refers to a non-coated paper mainlymade from pulp and used in printers or the like. More specifically,plain papers include woodfree paper defined as No 6074 in JIS P 0001,PPC paper defined as No. 6139 in JIS P 0001, and other non-coatedprinting papers. Commercially available papers such as Xerox 4200(manufactured by Xerox) and GeoCycle (produced by Georgia-Pacific) maybe used as plain paper.

The ink jet recording apparatus will be described with reference to FIG.1, first. A line ink jet recording apparatus 100 capable of high-speedhigh density printing typically includes an ink jet head unit 190 thatejects droplets of ink compositions (hereinafter may be referred to asink droplets) onto a recording medium 101, such as plain paper, torecords images, a transport belt 130 that transports the recordingmedium 101 to a position under the ink jet head unit 190, anaccommodating cassette 104 in which the recording medium 101 isaccommodated, a paper feed roller 105 that feeds the recording medium101 from the accommodating cassette 104, a pair of transport rollers(gate rollers) 140 for transport the recording medium 101, a pair ofejection rollers 150 for ejecting the recording medium 101, a paperejection cassette 106 that receives the printed recording medium 101, acontrol section 111, and a position detecting sensor 109 that detectsthe position of the recording medium 101.

The ink jet head unit 190 includes a plurality of ink jet heads 110A,110B, 110C and 110D (or 110A to 110D, these reference numerals are notshown in FIG. 1) corresponding to the respective types of ink. Each inkjet head is a line head having many ink ejection nozzles arranged in thewidth direction of the recording medium 101 across the entire width.

The transport belt 130, which is a ring, transports the recording medium101 to the position of the ink jet head unit 190 (printing region). Adriving roller 180 drives the transport belt 130 and a driven roller 170is driven so as to oppose the transport belt 130 to ink-ejection portsof the ink jet head unit 190. The driving roller 180 is operated by amotor 115 controlled by the control section 111. The paper feed roller105 is intended to send the recording medium 101 in the accommodatingcassette 104 to the transport rollers 140, and are operated by a motor118 controlled by the control section 111.

The transport rollers 140 include a driving roller 140A acting as aroller unit operated by a motor 116 controlled by the control section111, and a driven roller 140B driven by contact with the driving roller140A. The ejection rollers 150 constitute an ejection roller pairincluding a driving roller 150A operated by a motor 117 controlled bythe control section 111, and a driven roller 150B driven by contact withthe driving roller 150A.

The control section 111 includes a CPU (central processing unit) thatperforms printing operation (recording operation) and other operations,a RAM (random access memory) module that stores printing data (recordingdata) transmitted from a host computer through an interface (IF) in adata storage region or temporarily stores other data, and a PROM orEEPROM (electrically erasable programmable read-only memory) module thatstores a control program or the like for controlling various portions.

A position-detecting sensor 109 is a reflective photosensor prepared by,for example, combining an IR emitting diode acting as a light-emittingdevice, and a phototransistor acting as a light-receiving element. Theposition detecting sensor 109 is disposed at a paper transport portionbetween the paper feed roller 105 and the transport rollers 140, anddetects the front end position of the transported recording medium 101(the presence or absence of the recording medium 101). The detectionsignal of the sensor is transmitted to the control section 111. Thecontrol section 111 controls the transport rollers 140 according to thedetection signal of the front end of the recording medium 101.

The recording medium 101 is transported to the transport rollers 140rotated by the motor 116 operated according to a driving signal from thecontrol section 111 to come into contact with the transport rollers 140.Consequently, the position and orientation of the front end of therecording medium 101 are aligned by the contact of the front end withthe transport rollers, so that the recording medium 101 is pinchedbetween the driving roller 140A and the driven roller 140B and sent ontothe transport belt 130. When the recording medium 101 is transported tothe printing region under the ink jet head unit 190 by the transportbelt 130, ink droplets are ejected onto the recording medium 101 beingtransported on the transport belt 130 from the nozzles of the ink jethead unit 190. Thus printing is performed according to printing data.

For printing on the recording medium 101, in the control section 111,the RAM module receives printing data from a host computer through theinterface, and the CPU processes the data in a predetermined manner.According to the processed data, a driving signal is outputted to thehead driver and then inputted to the ink jet head unit 190.Consequently, an electrostatic actuator to which the driving signal hasbeen inputted operates so that ink droplets are ejected to print(record) an image according to the printing data onto the recordingmedium 101 through the corresponding nozzles.

The printed recording medium 101 is transported to the ejection portion(ejection rollers 150) by the transport belt 130. When the transportedrecording medium 101 has been reached the ejection rollers 150, themotor 117 rotates the driving roller 150A according to a driving signalfrom the control section 111, and the recording medium 101 is pinchedbetween the driving roller 150A and the driven roller 150B rotated bythe contact with the driving roller 150A and thus transported into thepaper ejection cassette 106.

Turning to FIGS. 2 and 3, the present embodiment will be furtherdescribed. The ink jet recording apparatus 100 includes the ink jet headunit 190, a platen portion 120 disposed under the ink jet head unit 190so as to oppose each other, a recording medium feed portion (not shown)that feeds the recording medium 101 at the upstream side of the platenportion 120 in the transport direction, a recording medium receivingportion (not shown) that receives the printed recording medium 101 atthe downstream side of the platen portion 120 in the transportdirection, and a transport unit 160 that transports the recording medium101 over the platen portion 120 from the recording medium feed portionand further transports the recording medium 101 to the recording mediumreceiving portion after being printed.

The ink jet head unit 190 includes a plurality of lines of ink jet heads110A to 110D having ink nozzles. Each line includes a plurality of thesame ink jet heads so that the ink nozzles are arranged in a line. Theink jet heads 110A to 110D are arranged in a staggered manner in such amanner that each line of the same ink jet heads extends in a directionperpendicular to the recording medium-transport direction (in the widthdirection of the recording medium 101) for line printing.

The two lines of the ink jet heads 110A and 110B eject droplets of thesame first ink composition, and the other liens, the two lines of theink jet heads 110C and 110D, eject droplets of the same second inkcomposition. The color of the first ink composition ejected from the inkjet heads 110A and 110B is different from the color of the second inkcomposition ejected from the ink jet heads 110C and 110D. The ink jetheads 110A and 110B are overlapped with each other when viewed from therecording medium-transport direction so that the first ink compositioncan form an image at any position across the width of the recordingmedium. The ink jet heads 110C and 110D are also arranged in the samemanner.

Preferably, the distance between the line of the ink jet heads 110A andthe line of the ink jet heads 110B is the same as the distance betweenthe line of the ink jet head 110B and the line of the ink jet heads110C, and/or the distance between the line of the ink jet heads 110C andthe line of the ink jet heads 110D is the same as the distance betweenthe ink jet heads 110B and the line of the ink jet heads 110C. In theknown technique, this arrangement is difficult. The arrangement in whichthe distance between ink jet head lines of different colors isapproximately the same as the distance between ink jet head lines of thesame color causes color bleeding particularly in high speed printing. Onthe other hand, the present embodiment allows the ink jet heads to bearranged in such a manner that the distance between the ink jet headlines of different colors is smaller than or equal to the distancebetween the ink jet head lines of the same color, because the embodimentcan reduce the occurrence of color bleeding, as described below. Inaddition, since the length of the entire ink jet head unit 190 in thetransport direction can be reduced, the apparatus can be reduced insize, weight, and cost. Furthermore, in the present embodiment, thedisplacement of dots in the width direction, which occurs when therecording medium 101 is skewed in its width direction, can be minimizedby reducing the distance between the lines of different color heads.

The platen portion 120 includes a transport belt 130 on which therecording medium 101 is transported. The transport belt 130 doubles as aplaten belt. The transport unit 160 includes the transport belt 130 andthe pairs of transport rollers 140 and ejection rollers 150 respectivelydisposed at the upstream side and the downstream side of the transportbelt 130 in the transport direction. The rollers of each pair opposeeach other so as to pinch the recording medium 101 from the verticaldirection. The transport belt 130 is operated so as to transport therecording medium 101 thereon in the transport direction by the rotationof the driven roller 170 and the driving roller 180.

The other portion of the ink jet recording apparatus 100 may have thesame structure as the known apparatus.

The operation of the ink jet recording apparatus 100, that is, the inkjet recording method, is performed as below. First, the transport unit160, that is, the transport belt 130 and the transport rollers 140 and150, are operated to transport the recording medium 101 in the transportdirection from the medium feed portion to the platen portion 120. Whenthe recording medium 101 has transported to the position under the inkjet heads 110A and 110B, droplets of the first ink composition areejected from the nozzles of the ink jet heads 110A and 110B. Thedroplets are landed on desired positions on the printing surface (topsurface) of the recording medium 101 where images are to be formed.Subsequently, when the recording medium 101 has transported to theposition under the ink jet heads 110C and 110D, droplets of the secondink composition are ejected from the nozzles of the ink jet heads 110Cand 110D. The droplets are landed on desired positions on the printingsurface (top surface) of the recording medium 101 where images are to beformed. At this time, part of the second ink composition may landdirectly on the printing surface of the recording medium 101, and atleast part of the second ink composition lands on the image formed withthe first ink composition. Thus an image is formed on the recordingmedium 101. The recording medium 101 on which the image has been formed(printed) is transported to the medium receiving portion disposeddownstream from the ink jet head unit 190.

The technique as described above is hereinafter called “color completionmethod”, in which a color ink composition is deposited on all theportions where an image of this color is to be formed, and subsequentlyanother color ink composition is ejected on all the positions whereanother image of this color is to be formed.

In the ink jet recording method of the present embodiment, an inkcomposition is ejected as droplets from fine nozzles and deposited on arecording medium. Techniques for this method will now be described indetail.

A first technique is electrostatic suction. In this technique, a strongelectric field is applied between a nozzle and an acceleration electrodedisposed in front of the nozzle so that ink droplets are continuouslyejected from the nozzle. A printing information signal is applied todeflecting electrodes while the droplets fly between deflectingelectrodes, and recording is thus performed. The droplets may bedeposited according to the printing information signal withoutdeflecting the ink droplets.

A second technique is a method for forcibly ejecting ink droplets byapplying a pressure to a liquid ink composition with a small pump andmechanically vibrating the nozzle with a quartz resonator or the like.The ink droplets are charged simultaneously with being ejected, andrecording is performed by applying a printing information signal to thedeflecting electrodes while the ink droplets fly between the deflectingelectrodes.

A third technique uses a piezoelectric element. A pressure and aprinting information signal are simultaneously applied to a liquid inkcomposition by the piezoelectric element. Thus recording is performed byejecting ink droplets for recording.

In a fourth technique, the volume of the liquid ink composition israpidly expanded by thermal energy. The ink composition is bubbled bybeing heated with a small electrode according to a printing informationsignal, and is thus ejected for recording.

Any of the above techniques can be applied to the ink jet recordingmethod of the present embodiment.

The ink compositions (first and second ink compositions) used in the inkjet recording method of the present embodiment will now be described.

From the viewpoint of safety and handling, aqueous ink compositionsmainly containing water as the main solvent are preferably used in thepresent embodiment. The water is preferably pure water or ultra purewater, such as ion exchanged water, ultrafiltered water, reverse osmosiswater, or distilled water. In particular, the water is preferablysterilized by, for example, UV irradiation or addition of hydrogenperoxide. The use of sterile water can prevent the occurrence of mold orbacteria and thus allows long-term storage. From the viewpoint ofensuring appropriate physical properties (yield value, viscosity, etc.),stability and reliability of the ink composition, it is preferable thatthe ink composition contain 10% to 60% by mass of water.

By controlling the water content in the ink composition in the aboverange, the amount of water absorbed to the cellulose of plain paper isreduced. Accordingly, the swelling of the cellulose, which is consideredto be a cause of cockling and curling, can be prevented. The propertiesof preventing cockling and curling are referred to as anti-cocklingproperty and anti-curling property, respectively.

If the water content is less than 10% by mass, the fixability to therecording medium may be reduced. In contrast, if the water content ismore than 60% by mass, cockling or curling is liable to occur as in useof known aqueous ink compositions, when printing is performed on arecording medium having an absorption layer on a paper support thatcannot absorb ink much.

The viscosity of the ink composition at a temperature in the range of 10to 40° C. is varied depending on the temperature dependences of thecoloring agent, moisturizing agent, solvent and other constituents inthe ink composition. Among these constituents, the moisturizing agenthas a large effect, and tends to increase the viscosity at 10° C. and toreduce the viscosity at 40° C., depending on the material and the amountadded or content. In the description herein, when the difference inviscosity between temperatures of 10° C. and 40° C. is small, it is saidthat the ink composition has a good viscosity property with temperature.

Preferably, the ink composition used in the present embodiment containsat least one moisturizing agent selected from the group consisting ofthe following groups (A), (B) and (C), from the viewpoint of maintaininga suitable balance among the anti-cockling property, the anti-curlingproperty, the strike-through property, the anti-clogging property, andthe viscosity property with temperature. Moisturizing agent (A) is atleast one compound selected from group (A) consisting of glycerin,1,2,6-hexanetriol, diethylene glycol, triethylene glycol, tetraethyleneglycol and dipropylene glycol. Moisturizing agent (B) is at least onecompound selected from group (B) consisting of trimethylolpropane andtrimethylolethane. Moisturizing agent (C) is at least one compoundhaving a molecular weight in the range of 100 to 200 selected from group(C) consisting of betaines, saccharides and urea compounds.

Moisturizing agent (A) is effective particularly in suppressingclogging, and also in suppressing curling and cockling. Thismoisturizing agent however can penetrate the recording medium, and isaccordingly inferior in strike-through property. From the viewpoint ofensuring the above advantage, glycerin and triethylene glycol arepreferred as moisturizing agent (A).

Moisturizing agent (B) is effective in suppressing clogging and issuperior in strike-through property because it has the effect ofsuppressing penetration. From the viewpoint of ensuring theseadvantages, trimethylolpropane is preferred as moisturizing agent (B).

Moisturizing agents (A) and (B) each have a large difference between theviscosities at temperatures of 10° C. and 40° C. Accordingly theviscosity property with temperature of the ink composition is moresignificantly affected and, thus, the viscosity of the ink compositionhas a large difference between temperatures of 10° C. and 40° C., as thecontent of the moisturizing agents is increased in the ink composition.

Moisturizing agent (C) is superior in anti-curling property andanti-cockling property. This moisturizing agent is also superior inviscosity property with temperature. Examples of moisturizing agent (C)include betaines that are N-trialkyl-substituted compounds of aminoacids, such as glycine betaine (molecular weight: 117, may be referredto as trimethylglycine), γ-butyrobetaine (molecular weight: 145),homarine (molecular weight: 137), trigonelline (molecular weight: 137),carnitine (molecular weight: 161), homoserine betaine (molecular weight:161), valine betaine (molecular weight: 159), lysine betaine (molecularweight: 188), ornithine betaine (molecular weight: 176), alanine betaine(molecular weight: 117), stachydrine (molecular weight: 185), andbetaine glutamate (molecular weight: 189); saccharides, such as glucose(molecular weight: 180), mannose (molecular weight: 180), fructose(molecular weight: 180), ribose (molecular weight: 150), xylose(molecular weight: 150), arabinose (molecular weight: 150), galactose(molecular weight: 180), and sorbitol (molecular weight: 182); and ureacompounds, such as allylurea (molecular weight: 100), N,N-dimethylolurea(molecular weight: 120), malonylurea (molecular weight: 128),carbamylurea (molecular weight: 103), 1,1-diethylurea (molecular weight:116), n-butylurea (molecular weight: 116), creatinine (molecular weight:113), and benzylurea (molecular weight: 150). If the molecular weight ofmoisturizing agent (C) is less than 100, the difference between theviscosities at temperatures of 10° C. and 40° C. tends to be increased.On the other hand, if the molecular weight is 200 or more, the viscosityof the ink composition is likely to increase with respect to the contentof moisturizing agent (C) in the ink composition. Accordingly, themolecular weight of moisturizing agent (C) is preferably in the range of100 to 200. Among the above compounds, glycine betaine is particularlysuitable because it is highly effective in suppressing curling, and iscommercially available as, for example, AMINOCOAT from Asahi KaseiChemicals.

The total content of moisturizing agents (A), (B) and (C) in the inkcomposition is preferably in the range of 10% to 40% by mass, from theviewpoint of the anti-curling property, the anti-cockling property, thestrike-through property, and the anti-clogging property.

Preferably, the proportion of moisturizing agents on a mass basis is(A):(B):(C)=(1.0):(0.1 to 1.0):(1.0 to 3.5), from the viewpoint ofproducing the above-described advantageous effects of the moisturizingagent with a good balance. If the ink composition contains twomoisturizing agents selected from groups (A), (B) and (C), the massratio of moisturizing agents is preferably (A):(B)=(1.0):(0.1 to 1.0),(A):(C)=(1.0):(1.0 to 3.5), or (B):(C)=(1.0):(1.0 to 3.5), from the sameviewpoint as above. If the mass ratio of moisturizing agent (B) tomoisturizing agent (A) is higher than the above ratio, the anti-curlingproperty and anti-cockling property are degraded. If it is lower thanthe above ratio, the strike-through property is degraded. If the massratio of moisturizing agent (C) to moisturizing agent (A) is higher thanthe above ratio, the anti-clogging property is degraded. If it is lowerthan the above ratio, it becomes difficult particularly to prevent thenonuniformity in image density, and the anti-curling property andanti-cockling property are degraded. If the mass ratio of moisturizingagent (C) to moisturizing agent (B) is higher than the above ratio, theanti-clogging property is degraded. If it is lower than the above ratio,it becomes difficult to control the nonuniformity in image density, andthe anti-curling property and anti-cockling property are degraded.

Preferably, the ink composition used in the present embodiment containsa water-soluble organic solvent in order to prevent clogging in thevicinity of the nozzles of the ink jet head, to control the penetrationand bleeding of the ink composition into the recording medium, and tomake the ink composition easy to dry. Accordingly, the water-solubleorganic solvent preferably contains 1,2-alkanediol and/or glycol ether.Examples of 1,2-alkanediol include 1,2-octanediol, 1,2-hexanediol,1,2-pentanediol, and 4-methyl-1,2-pentanediol. Examples of glycol etherinclude ethylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monobutyl ether, ethylene glycol monomethyl etheracetate, diethylene glycol monomethyl ether, diethylene glycol monoethylether, diethylene glycol mono-n-propyl ether, ethylene glycolmono-iso-propyl ether, diethylene glycol mono-iso-propyl ether, ethyleneglycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether,diethylene glycol mono-t-butyl ether, triethylene glycol mono-n-butylether (TEGmBE), 1-methyl-1-methoxy butanol, propylene glycol monomethylether, propylene glycol monoethyl ether, propylene glycol mono-t-butylether, propylene glycol mono-n-propyl ether, propylene glycolmono-iso-propyl ether, dipropylene glycol monomethyl ether, dipropyleneglycol monoethyl ether, dipropylene glycol mono-n-propyl ether, anddipropylene glycol mono-iso-propyl ether. In addition, 2-pyrrolidone,N-methyl-2-pyrrolidone and the like can also be used as thewater-soluble organic solvent. These water-soluble organic solvents areused singly or in combination, and their total content in the inkcomposition is preferably 1% to 50% by mass, from the viewpoint ofensuring appropriate physical properties (yield value, viscosity, etc.)of the ink composition, and ensuring high print quality and reliability.

In order to control the wettability of the ink composition to therecording medium so as to ensure the penetration into the recordingmedium and the printing stability in the ink jet recording method, theink composition preferably contains a surface tension modifier.Preferred surface tension modifiers include acetylene glycol-basedsurfactants and polyether-modified siloxanes. Examples of the acetyleneglycol-based surfactant include Surfinols 420, 440, 465, 485 and 104 andSurfinol STG (each product name, produced by Air Products), and OlfinesPD-001, SPC, E1004 and E1010 (each product name, produced by NissinChemical Industry), and Acetylenols E00, E40, E100 and LH (each productname, produced by Kawaken Fine Chemical). Examples of thepolyether-modified siloxane include BYK-346, BYK-347, BTK-348 and BYK-UV3530 (each produced by BYK). These surface tension modifiers can be usedsingly or in combination in the ink composition, and are contained insuch an amount as can control the surface tension of the ink compositionin the range of 20 to 40 mN/m, and preferably contained in an amount of0.1% to 3.0% by mass in the ink composition.

The ink composition may contain a pH adjuster, a complexing agent, anantifoaming agent, an antioxidant, an ultraviolet light absorbent, apreservative, an antifungal agent and other additives, if necessary.Examples of the pH adjuster include alkali metal hydroxides, such aslithium hydroxide, potassium hydroxide, and sodium hydroxide; andammonia and alkanolamines, such as triethanolamine, tripropanolamine,diethanolamine, and monoethanolamine. Preferably, the ink composition isadjusted to a pH of 6 to 10 by adding at least one pH adjuster selectedfrom the group consisting of alkali metal hydroxides, ammonia,triethanolamine, and tripropanolamine. If the pH of the ink compositionis outside this range, the materials of the ink jet printer are likelyto be adversely affected, and the printer becomes difficult to recoverfrom clogging.

The pigment used in the ink composition used in the present embodimentmay be a known inorganic or organic pigment. Examples of such a pigmentinclude Pigment Yellows, Pigment Reds, Pigment Violets, Pigment Bluesand Pigment Blacks that can be designated by color index numbers, andalso include phthalocyanine-based pigments, azo-based pigments,anthraquinone-based pigments, azomethine-based pigments, and pigmentshaving a condensed ring. Other pigments may also be used, includingorganic pigments, such as Yellow Nos. 4, 5, 205 and 401, Orange Nos. 228and 405, and Blue Nos. 1 and 404; and inorganic pigments, such as carbonblack, titanium oxide, zinc oxide, zirconium oxide, iron oxide,ultramarine blue, iron blue, and chromium oxide. Pigments designated bycolor indexes include C. I. Pigment Yellows 1, 3, 12, 13, 14, 17, 24,34, 35, 37, 42, 53, 55, 74, 81, 83, 95, 97, 98, 100, 101, 104, 108, 109,110, 117, 120, 128, 138, 150, 153, 155, 174, 180 and 198, C. I. PigmentReds 1, 3, 5, 8, 9, 16, 17, 19, 22, 38, 57:1, 90, 112, 122, 123, 127,146, 184 and 202, C. I. Pigment Violets 1, 3, 5:1, 16, 19, 23 and 38, C.I. Pigment Blues 1, 2, 15, 15:1, 15:2, 15:3, 15:4 and 16, and C. I.Pigment Blacks 1 and 7. These pigments may be contained singly or incombination in the ink composition.

The pigment used in the present embodiment is preferably dispersed inresin from the viewpoint of controlling the yield value so as to have anappropriate relationship with the yield values of other inkcompositions. Accordingly, the pigment is preferably added to the inkcomposition as a pigment-dispersed liquid. The pigment-dispersed liquidmay be prepared by dispersing a pigment with a dispersant, such as apolymer dispersant or a surfactant, in an aqueous medium using a ballmill, a roll mill, a bead mill, a high-pressure homogenizer, ahigh-speed agitating disperser or the like. A self-dispersing pigmentprepared by binding a group that can impart dispersion characteristics(hydrophilic functional group and/or its salt) to the surfaces of thepigment particles directly or with an alkyl, alkyl ether or aryl groupor the like therebetween may be dispersed or dissolved in an aqueousmedium without using a dispersant. The pigment-dispersed liquid thusprepared is added to the ink composition. Preferably, apigment-dispersed liquid in which a self-dispersing pigment is dispersedin an aqueous medium is used from the viewpoint of controlling the yieldvalue so as to have an appropriate relationship with the yield values ofother ink compositions.

Examples of the polymer dispersant include natural polymer dispersants,such as glue, gelatin and saponin; and synthetic polymer dispersants,such as polyvinyl alcohols, polypyrrolidones, acrylic resins(polyacrylic acid, acrylic acid-acrylonitrile copolymer, vinylacetate-acrylic acid copolymer, vinyl acetate-acrylic ester copolymer,etc.), styrene-acrylic acid resins (styrene-acrylic acid copolymer,styrene-methacrylic acid copolymer, styrene-methacrylic acid-acrylicacid alkyl ester copolymer, styrene-α-methylstyrene-acrylic acidcopolymer, styrene-α-methylstyrene-acrylic acid-acrylic acid alkyl estercopolymer, styrene-vinyl acetate-acrylic acid copolymer, etc.),styrene-maleic acid resins, vinyl acetate-fatty acid vinyl-ethylenecopolymers, and salts of these resins. These copolymers may be ofrandom, block or graft type.

Surfactants that can be used as the dispersant include anionicsurfactants, such as fatty acid salts, higher alkyl dicarboxylic acidsalts, higher alcohol sulfates, and higher alkyl sulfonates; cationicsurfactants, such as fatty acid amine salts and fatty acid ammoniumsalts; and nonionic surfactants, such as polyoxyalkyl ethers,polyoxyalkyl esters, and sorbitan alkyl esters.

Among those dispersants, water-insoluble resins are particularlypreferred. Preferably, an exemplary water-insoluble dispersant is ablock copolymer of a monomer having a hydrophobic group and a monomerhaving a hydrophilic group (hydrophilic functional group), including agroup capable of forming a salt, and having a solubility of less than 1g in 100 g of water at 25° C. after neutralization. Examples of themonomer having a hydrophobic group include methacrylic acid esters, suchas methyl methacrylate, ethyl methacrylate, isopropyl methacrylate,n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate,isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate,octyl methacrylate, decyl methacrylate, dodecyl methacrylate, octadecylmethacrylate, cyclohexyl methacrylate, phenyl methacrylate, benzylmethacrylate, and glycidyl methacrylate; vinyl esters, such as vinylacetate; vinyl cyanides, such as acrylonitrile and methacrylonitrile;and aromatic vinyl monomers, such as styrene, α-methylstyrene,vinyltoluene, 4-t-butylstyrene, chlorostyrene, vinyl anisole, andvinylnaphthalene. These monomers may be used singly or in combination.Examples of the monomer having a hydrophilic group include polyethyleneglycol monomethacrylate, polypropylene glycol monomethacrylate, andethylene glycol-propylene glycol monomethacrylate. These monomers may beused singly or in combination. Examples of the monomer having a groupcapable of forming a salt include acrylic acid, methacrylic acid,styrene-carboxylic acid, and maleic acid. These monomers may be usedsingly or in combination. In addition, macromonomers, whose one end hasa polymerizable functional group, such as styrene-based macromonomersand silicone-based macromonomers, and other monomers may be combined.

The water-insoluble resin is preferably used in form of a salt that hasbeen neutralized with an alkaline neutralizer, such as ethylamine, atertiary amine such as trimethylamine, lithium hydroxide, sodiumhydroxide, potassium hydroxide, or ammonia, and preferably has a weightaverage molecular weight of about 10,000 to 150,000 from the viewpointof stably dispersing the pigment.

The self-dispersing pigment, which can be dispersed or dissolved inwater without using a dispersant, can be prepared by, for example, beingsubjected to physical treatment or chemical treatment for binding(grafting) a group capable of imparting dispersion characteristics or anactive species having a group capable of imparting dispersioncharacteristics to the surfaces of the pigment particles. For thephysical treatment, vacuum plasma treatment may be performed. Thechemical treatment may be performed by, for example, wet oxidation inwhich the surfaces of the pigment particles are oxidized with anoxidizing agent in water, or a process in which a compound having aphenyl group and at least two hydrophilic groups is bound to thesurfaces of the pigment so that the hydrophilic groups are bound to thesurfaces of the pigment with the phenyl group therebetween. For example,the compound having a phenyl group and at least two hydrophilic groupsmay be p-aminobenzoic acid or sulfanilic acid. If p-aminobenzoic acid isused, its carboxyl group is bound to the surfaces of the pigment withthe phenyl group therebetween. If sulfanilic acid is used, its sulfoxygroup or a salt with its sulfoxyl group (for example, sodium persulfateor a sodium salt derived from sodium persulfate) is bound to thesurfaces of the pigment with a phenyl group therebetween. Among these,self-dispersing pigments whose surfaces are bound with a hydrophilicgroup with a phenyl group therebetween are preferred from the viewpointof the stability in viscosity with time of the ink composition and theprevention of sedimentation resulting from the aggregation of thepigment.

Since ink compositions containing a self-dispersing pigment do notrequire a dispersant to disperse the pigment, the defoaming property ofthe ink composition is not degraded by a dispersant. Accordingly, theink composition is hardly foamed and is easy to prepare so as to have ahigh ejection stability. Also, since a significant increase in viscositycaused by a dispersant can be suppressed, the pigment content can beincreased to increase the print density, or the handling of the inkcomposition can be easy. Since self-dispersing pigments have theseadvantages, they are useful for black ink compositions, which arerequired to form dense images. The black ink composition used in thepresent embodiment preferably contains a self-dispersing pigment capableof being dispersed or dissolved in water without using a dispersant.

In the present embodiment, a self-dispersing pigment that can besurface-treated by oxidation with a hypohalous acid and/or hypohalousacid salt, a persulfate, or ozone is preferred from the viewpoint ofhigh color developability. By this surface treatment, a hydrophilicgroup is introduced to the self-dispersing pigment. Self-dispersingpigments that can be surface-treated by oxidation with a persulfate orozone, particularly self-dispersing pigments that can be surface-treatedby oxidation with ozone, are preferred from the viewpoint of (1)preventing the increase in viscosity of the ink composition when thematerials are compounded, (2) preventing sedimentation resulting fromthe aggregation of the pigment, and (3) maintaining the advantages of(1) and (2) for the long term. Commercially available self-dispersingpigments may be used. Exemplary commercially available self-dispersingpigments include Microjet CW-1 (product name, produced by OrientChemical Industries), and CAB-O-JET200 and CAB-O-JET 300 (each productname, produced by Cabot).

Preferably, the pigment in the ink composition has a volume averageparticle size in the range of 50 to 200 nm from the viewpoint of thestorage stability of the ink composition and the prevention of nozzleclogging. The volume average particle size can be measured withMicrotrac UPA 150 (manufactured by Microtrac) or a particle sizedistribution analyzer LPA 3100 (manufactured by Otsuka electronics).

Preferably, the ink composition contains 6% to 25% by mass of pigment.If the pigment content is less than 6% by mass, the print density (colordevelopability) can be insufficient. If the pigment content is more than25% by mass, problems with reliability may occur, such as nozzleclogging or unstable ejection.

Preferably, the ink composition used in the present embodiment containsa resin emulsion from the viewpoint of ensuring a fixability to recordedmatter. The resin emulsion preferably contains resin particles having aminimum film forming temperature of less than 20° C. By using resinparticles having a minimum film forming temperature of less than 20° C.as the resin emulsion, the resin particles can be formed into a film attemperatures (typically 20° C. or more) in use, and thus, the fixabilityof the ink composition to the recording medium and the rub fastness ofthe composition can be enhanced.

The minimum film forming temperature can be measured as below. First, aresin emulsion is applied at a thickness of 0.3 mm onto a stainlesssteel plate of a thermal gradient tester. The coated stainless steelplate is immediately placed on a plate in a basket containing silica geland covered with a transparent plastic cover. After the coating isdried, the temperature at a boundary between the portion of a uniformlyformed coating and the portion of a clouded coating is measured. Themeasured temperature is the minimum film forming temperature.

Preferably, the resin emulsion contains particles of at least one resinselected from the group consisting of acrylic resins, methacrylicresins, vinyl acetate resins, vinyl chloride resins, and styrene-acrylicresins. These resins may be homopolymer or copolymer, or have asingle-phase structure or a multi-phase (core-shell) structure.

Furthermore, it is preferable that at least any one of the resinemulsions added to the ink composition be an emulsion of resin particlesprepared by emulsion polymerization of an unsaturated monomer. If resinparticles are added singly to the ink composition, they may not besufficiently dispersed. It is preferable to add resin particles in formof emulsion from the viewpoint of the manufacture of the inkcomposition. From the viewpoint of the storage stability of the inkcomposition, acrylic resin emulsion is preferably used.

Resin emulsion such as acrylic resin emulsion can be prepared by a knownemulsion polymerization. For example, an unsaturated monomer, such asunsaturated vinyl monomer, can be subjected to emulsion polymerizationin water containing a polymerization initiator and a surfactant.

Unsaturated monomers conventionally used for emulsion polymerization canbe used as the unsaturated monomers, and examples of such an unsaturatedmonomer include acrylic ester monomers, methacrylic ester monomers,aromatic vinyl monomers, vinyl ester monomers, vinyl cyanide monomers,halogenated monomers, olefin monomers, and diene monomers.

More specifically, exemplary unsaturated monomers include acrylicesters, such as methyl acrylate, ethyl acrylate, isopropyl acrylate,n-butyl acrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate,n-hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, decyl acrylate,dodecyl acrylate, octadecyl acrylate, cyclohexyl acrylate, phenylacrylate, benzyl acrylate, and glycidyl acrylate; methacrylic esters,such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate,n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate,isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate,octyl methacrylate, decyl methacrylate, dodecyl methacrylate, octadecylmethacrylate, cyclohexyl methacrylate, phenyl methacrylate, benzylmethacrylate, and glycidyl methacrylate; vinyl esters, such as vinylacetate; vinyl cyanides, such as acrylonitrile and methacrylonitrile;halogenated monomers, such as vinylidene chloride and vinyl chloride;aromatic vinyl monomers, such as styrene, α-methylstyrene, vinyltoluene,4-t-butylstyrene, chlorostyrene, vinylanisole, and vinylnaphthalene;olefins, such as ethylene and propylene; dienes, such as butadiene andchloroprene; vinyl monomers, such as vinyl ether, vinyl ketone, andvinyl pyrrolidone; unsaturated carboxylic acids, such as acrylic acid,methacrylic acid, itaconic acid, fumaric acid, and maleic acid;acrylamide compounds, such as acrylamide, methacrylamide, andN,N′-dimethylacrylamide; and hydroxy group-containing monomers, such as2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethylmethacrylate, and 2-hydroxypropyl methacrylate. These unsaturatedmonomers may be used singly or in combination.

In addition, polymerizable crosslinking monomers having at least twodouble bonds may be used as the unsaturated monomer. Exemplarypolymerizable crosslinking monomers having at least two double bondsinclude diacrylate compounds, such as polyethylene glycol diacrylate,triethylene glycol diacrylate, 1,3-butylene glycol diacrylate,1,4-butylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentylglycol diacrylate, 1,9-nonanediol diacrylate, polypropylene glycoldiacrylate, 2,2′-bis(4-acryloxypropyloxyphenyl)propane, and2,2′-bis(4-acryloxydiethoxyphenyl)propane; triacrylate compounds, suchas trimethylolpropane triacrylate, trimethylolethane triacrylate, andtetramethylolmethane triacrylate; tetraacrylate compounds, such asditrimethylol tetraacrylate, tetramethylolmethane tetraacrylate, andpentaerythritol tetraacrylate; hexaacrylate compounds, such asdipentaerythritol hexaacrylate; dimethacrylate compounds, such asethylene glycol dimethacrylate, diethylene glycol dimethacrylate,triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate,1,3-butylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylate,1,6-hexanediol dimethacrylate, neopentyl glycol dimethacrylate,dipropylene glycol dimethacrylate, polypropylene glycol dimethacrylate,polybutylene glycol dimethacrylate, and2,2′-bis(4-methacryloxydiethoxyphenyl)propane; trimethacrylatecompounds, such as trimethylolpropane trimethacrylate andtrimethylolethane trimethacrylate; methylenebisacrylamide; anddivinylbenzene. These compounds may be used singly or in combination.

In addition to the polymerization initiator and surfactant used for theemulsion polymerization, a chain transfer agent, a neutralizer andothers may be used according to conventional processes. In particular,preferred neutralizers include ammonia, inorganic alkali metalhydroxides, such as sodium hydroxide and potassium hydroxide.

In the present embodiment, it is preferable that the resin emulsion beadded so that the resin particle content in the ink composition is inthe range of 1% to 10% by mass, from the viewpoint of ensuring physicalproperties of the ink composition suitable for the ink jet method,reliability (anti-clogging property and ejection stability) andfixability of the ink composition.

Preferably, the resin emulsion in the ink composition has a volumeaverage particle size of 20 to 200 nm from the viewpoint of thedispersion stability of the resin particles in the ink composition.

In the ink jet recording method of the present embodiment, the yieldvalue of the second ink composition ejected from the ink jet heads 110Cand 110D is set so as to be higher than that of the first inkcomposition ejected from the ink jet heads 110A and 110B. In order tocontrol the yield values of the ink compositions as above, preferably,an ink composition containing a pigment having high color developabilityin a low content is used as the first ink composition, and an inkcomposition containing a pigment having low color developability in ahigh content is used as the second ink composition. To control the yieldvalues of the ink composition, alternatively, a surfactant, a dispersantor a rheology controlling agent may be added to the ink composition, orthe content of theses additives may be adjusted. The rheologycontrolling agent contains inorganic particles exhibiting structuralviscosity, such as colloidal silica, or contains a component insolubleor hardly soluble in a solvent such as modified urea and urea-modifiedurethane. Commercially available rheology controlling agents include,for example, BYK rheology controlling agents, such as BYK-405, BYK-420,BYK-425 and BYK-428 (each product name).

When the yield values of the first and second ink compositions arecontrolled as above, the first ink composition ejected from the ink jetheads 110A and 110B onto the recording medium 101 exhibits a relativelyhigh fluidity immediately after landing on the recording medium 101.However, the fluidity of the image of the first ink composition has beenreduced when the second ink composition is ejected from the ink jetheads 110C and 110D and deposited on the recording medium. Therefore,even if the second ink composition lands on the image in such a state,color bleeding of the first ink composition to the second inkcomposition can be suppressed because of the low fluidity of the firstink composition. Since the second ink composition originally has a highyield value and its fluidity is low even immediately after landing,color bleeding of the second ink composition to the first inkcomposition can also be suppressed. Thus, the ink jet recording methodof the present embodiment can prevent the occurrence of color bleeding.Furthermore, since the yield values are controlled as above, colorbleeding can be prevented even if the viscosities of the inkcompositions are comparable to each other. Accordingly, the ejectionproperty of ink droplets from the nozzles can be controlled to be thesame among the ink jet heads 110A to 110D, and the occurrence ofclogging can be suppressed.

If the yield value of the first ink composition that is previouslyejected is lower, the first ink composition is spread to some extent outof the portion intended to be printed. However, the undesirably spreadportion of the first ink composition can be hidden by depositing thesecond ink composition there. Since the second ink composition has ahigher yield value and accordingly does not much spread after landing,it can be selectively printed only on a desired portion. Furthermore,even if the print position of the first ink composition is displaced,the first ink composition spreads to the position intended to beprinted, after landing. The displaced position of the first inkcomposition can be canceled so as to be obscure by printing the secondink composition without displacement.

In the present embodiment, the first ink composition and the second inkcomposition are printed by the color completion method. The colorcompletion method is performed in such a manner that the recordingmedium 101 and the ink jet heads 110A and 110B are relatively moved in asingle pass, and the same color ink composition is not deposited on thesame position. Accordingly, this method has the advantages of: (1)allowing high-speed printing; and (2) allowing ink jet heads and theirunit to be reduced in size and weight to reduce the cost, because thereis no need of ink jet heads or nozzle lines for depositing the same inkcomposition several times on the same position.

In the ink jet recording method of the present embodiment, preferably,the image formed with the first ink composition has a higher lightnessthan the image formed with the second ink composition. Although colorbleeding is suppressed in the present embodiment, color bleedinggenerally can occur more from the first ink composition than from thesecond ink composition. Accordingly, if the lightness of the imageformed with the first ink composition is increased more, color bleedingfrom the first ink composition, even if occurs, can be canceled to theextent that it is not distinguished. In general, ink compositions havinglow lightnesses are required to form finer images. Since the second inkcomposition having a lower lightness has a higher yield value, the image(lines and characters) of the second ink composition is advantageouslysuppressed from spreading after landing, and thus can form a fine image.Ink compositions having low lightnesses such as a black ink compositionare often printed independently on a recording medium such as plainpaper. By controlling the lightness of the second ink composition havinga higher yield value, feathering can be prevented when the second inkcomposition is independently printed.

In a combination of the first ink composition and the second inkcomposition, the first ink composition may have a color other thanblack, and the second ink composition has black color. In anothercombination, a magenta or yellow ink composition may be used as thefirst ink composition, and a cyan ink composition may be used as thesecond ink composition. A cyan or yellow first ink composition and amagenta second ink composition may be combined.

The ink jet recording method of the present invention produces arecorded material. This recorded material has a sharp image withoutcolor bleeding because of the advantages of the ink jet method of thepresent embodiment. The image of the recorded material has few missingdots because it has been formed by depositing ink compositions havingstable fixability and not causing clogging on a recording medium, asintended. In the recorded material, the ink is safe and stable, and thequality of the record can be maintained on various types of recordingmedium independently of temperature in use. If plain paper is used, therecorded material is superior in anti-curling, anti-cockling andstrike-through property, and may have images on both sides of therecording material.

Thus, an embodiment of the invention has been described. The inventionis not limited to the embodiment disclosed above. Also, variousmodifications may be made without departing from the scope and spirit ofthe invention. Although the above embodiment has disclosed a structurein which ink jet heads 110A to 110D are arranged in a staggered manner,the ink jet heads may be arranged in a different manner. However it ispreferable that the ink jet heads be arranged in a staggered manner soas to partially overlap with each other when viewed in the transportdirection, because such an arrangement allows printing without a spacebetween each color line.

Although the above embodiment has disclosed an ink jet recording methodusing two color ink compositions, the method of another embodiment mayuse three or more color ink compositions, such as three, four, five orsix color ink compositions. In such a case, it is preferable that aplurality of ink jet heads for other colors are disposed downstream fromthe inkjet head 110D in the same manner as the ink jet heads 110A and110B.

If the ink compositions include four color ink compositions of black,cyan, magenta and yellow, preferably, they are arranged in decreasingorder of lightness from the upstream side of the transport direction,for example, in order of yellow ink composition, magenta inkcomposition, cyan ink composition and black ink composition. In thisinstance, preferably, the yield values of the ink compositions satisfythe relationship Y_(K)>Y_(C)>Y_(M)>Y_(Y). The reason of this is the sameas in the above embodiment using two color ink compositions. In therelationship, Y_(K), Y_(C), Y_(M) and Y_(Y) represent the yield valuesof the black ink composition, the cyan ink composition, the magenta inkcomposition and the yellow ink composition, respectively.

Alternatively, the four color ink compositions may be arranged indecreasing order of lightness from the upstream side of the transportdirection, for example, in order of yellow ink composition, cyan inkcomposition, magenta ink composition and black ink composition. In thisinstance, preferably, the yield values of the ink compositions satisfythe relationship Y_(K)>Y_(M)>Y_(C)>Y_(Y). The reason of this is the sameas in the above embodiment using two color ink compositions. In therelationship, Y_(K), Y_(M), Y_(C) and Y_(Y) represent the yield valuesof the black ink composition, the magenta ink composition, the cyan inkcomposition and the yellow ink composition, respectively.

EXAMPLES

The invention will be further described in detail with reference toExamples. The invention is however not limited to the examples.

Preparation of Coloring Agent

Self-Dispersing Pigment to which a Hydrophilic Group is Bound with aPhenyl Group Therebetween

Pigment-Dispersed Liquid CA

A 4 L stainless steel beaker was placed on a rotor-stator type highshear mixer L4RT-A (product name, manufactured by SILVERSON) andimmersed in an ice bath. The beaker was charged with about 75 g of C. I.Pigment Blue 15:4 and 1000 g of water, and the materials were mixed tobe homogenized by stirring at 7200 rpm for 15 minutes. To the mixturewere added 20 mL of a solution of 2.07 g (0.01 mol) of o-acetaniside inisopropanol, followed by stirring for 15 minutes.

In another vessel, a diazonium salt was produced by mixing 4.35 g (0.025mol) of sulfanilic acid, 30 mL of 1 N HCl, and 1.73 g (0.025 mol) ofsodium nitrite at 5 to 10° C. The diazonium salt was added to themixture of C. I. Pigment Blue 15:4 and o-acetaniside with stirring, andthe temperature was maintained at about 10° C. The resulting mixture wasadjusted to a pH of 5 to 6 by dropping 5 M sodium hydroxide solution,and was stirred for 2 hours while the progress of the reaction was beingchecked according to the presence or absence of the diazonium salt. Ifthe diazonium salt is present, when one droplet each of the reactionmixture and 1 M Na₂CO₃ solution containing 0.1% of aminosalicylic acidhas been dropped on filter paper, the two droplets come into contactwith each other and turn orange.

The mixture was placed in a Telsonic flow-type ultrasonic apparatus andsubjected to ultrasonic treatment for 2 hours. The resultingpigment-dispersed liquid was purified through a 50 nm diafiltrationmembrane column, and concentrated to a solid content of 20% by mass toyield cyan pigment-dispersed liquid CA.

Pigment-Dispersed Liquid MA

Magenta pigment-dispersed liquid MA containing 20% by mass of solidcontent was prepared in the same manner as in the preparation of cyanpigment-dispersed liquid CA, except that C. I. Pigment Red 122 was usedas the pigment instead of C. I. Pigment Blue 15:4.

Pigment-Dispersed Liquid YA

Yellow pigment-dispersed liquid YA containing 20% by mass of solidcontent was prepared in the same manner as in the preparation of cyanpigment-dispersed liquid CA, except that C. I. Pigment Yellow 74 wasused as the pigment instead of C. I. Pigment Blue 15:4.

Pigment-Dispersed Liquid KC

Black pigment-dispersed liquid KC containing 20% by mass of solidcontent was prepared in the same manner as in the preparation of cyanpigment-dispersed liquid CA, except that C. I. Pigment Black 7 (carbonblack) was used as the pigment instead of C. I. Pigment Blue 15:4.

Self-Dispersing Pigment to which Hydrophilic Group has been Bound byOxidation with Persulfate

Pigment-Dispersed Liquid KA

To 3 L of 2 N sodium persulfate solution was added 150 g of a carbonblack, Color Black S170 (product name, produced by Degussa), and thecarbon black was oxidized by stirring the mixture at an agitation speedof 1 s⁻¹ at 60° C. for 10 hours. The oxidized carbon black was filteredthrough an ultrafiltration membrane AHP-1010 (manufactured by AsahiKasei) to remove residual salts. Then, an aqueous solution of sodiumhydroxide was added to adjust the pH to 8. Subsequently, ultrafiltrationwas performed again for purification by removing excess salts and forconcentration by removing water. In this operation, the carbon blackcontent was adjusted so that the solution after treatment would contain20% by mass of carbon black. Thus black pigment-dispersed liquid KA wasprepared.

Self-Dispersing Pigment to which Hydrophilic Group has been Bound byOxidation with Ozone

Pigment-Dispersed Liquid KB

To 500 g of water was added 20 g of a carbon black, Color Black S170(product name, produced by Degussa). The mixture was dispersed in a homemixer for 5 minutes. To the resulting dispersion was dropped 1400 g ofsodium hypochlorite (effective chlorine concentration: 12%). The mixturewas subjected to reaction for 5 hours while being pulverized in a ballmill and further boiled for 4 hours for wet oxidation. The resultingliquid was filtered through a glass fiber filter GA-100 (product name,available from Advantech Toyo), followed by washing with water. Theresulting wet cake was dispersed in 5 kg of water and purified bydeionization through a reverse osmosis membrane until the electricconductivity was reduced to 2 mS/cm. Further, the dispersion wasconcentrated to a pigment concentration of 20% by mass to yieldpigment-dispersed liquid KB.

The volume average particle size of the pigment in this dispersionliquid was measured with Microtrac UPA 150 (produced by Microtrac). Theresult was 110 nm.

Pigment-Dispersed Liquid CB

Cyan pigment-dispersed liquid CB containing 20% by mass of solid contentwas prepared in the same manner as in the preparation of blackpigment-dispersed liquid KB, except that C. I. Pigment Blue 15:4 wasused as the pigment instead of carbon black.

Pigment-Dispersed Liquid YB

Yellow pigment-dispersed liquid YB containing 20% by mass of solidcontent was prepared in the same manner as in the preparation of blackpigment-dispersed liquid KB, except that C. I. Pigment Yellow 74 wasused as the pigment instead of carbon black.

Preparation of Ink Composition

Constituents of each ink composition were mixed in a proportion shown inTable 1, and the mixture was filtered through a membrane filter of 10 μmin pore size to yield an ink composition. Each content shown in Table 1is on the percent by mass basis, and “balance” in the row of ionexchanged water means that ion exchanged water was added to a total of100% by mass. “Newrex Soft 5S” is a surfactant produced by NOFCorporation.

TABLE 1 Cyan ink Magenta ink composition composition Yellow inkcomposition Black ink composition C1 C2 M1 Y1 Y2 Y3 K1 K2 K3 Cyanpigment-dispersed 50 — — — — — — — — liquid CA Cyan pigment-dispersed —50 — — — — — — — liquid CB Magenta pigment- — — 50 — — — — — — dispersedliquid MA Yellow pigment-dispersed — — — 50 — — — — — liquid YA Yellowpigment-dispersed — — — — 50 50 — — — liquid YB Black pigment-dispersed— — — — — — 35 — — liquid KA Black pigment-dispersed — — — — — — — 35 —liquid KB Black pigment-dispersed — — — — — — — — 35 liquid KC Glycerin8 8 8 8 8 8 8 8 8 Triethylene glycol 5 5 5 5 5 5 5 5 5 1,2-Hexanediol 55 5 5 5 5 5 5 5 Trimethylolpropane 3 3 3 3 3 3 3 3 3 TEGmBE 15 15 15 1515 15 20 20 15 Olfine E1010 0.5 0.5 0.5 0.5 0.5 — 0.5 0.5 0.5 Surfinol104 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Newrex soft 5S — — — — — 0.5 — —— Triethanolamine 1 1 1 1 1 1 1 1 1 Ion exchanged water Balance BalanceBalance Balance Balance Balance Balance Balance Balance Pigment contentin ink 10 10 10 10 10 10 7 7 7 composition Water content in ink 52 52 5252 52 52 50 50 55 compositionPreparation of Ink Set

The ink compositions prepared above were combined as shown in Table 2 toprepare ink sets.

TABLE 2 Transfer direction Upstream→Downstream Example 1 Ink set 1 Y1 M1C1 K1 Example 2 Ink set 2 Y2 C2 M1 K2 Comparative Ink set 3 Y3 M1 C2 K3ExampleEvaluation of Ink CompositionsTest 1: Yield Value

Each ink composition was placed in a cone/plate (diameter: 75 mm, angle:1°) attached to a viscoelasticity analyzer Physica MCR301 (product name)manufactured by Anton Paar, and the shear viscosity of the inkcomposition was measured at 20° C. with respect to the shear rate (10 to1000 s⁻¹). The obtained shear viscosity was applied to the Cassonequation to calculate the yield value. The results are shown in Tables 3to 5.

Test 2: Lightness

Each ink set was separately loaded in a line ink jet printer having thesame structure as the printer shown in FIGS. 1 to 3 except that an inkjet head unit capable of using four color ink compositions was used, anda patch pattern (solid image) was printed at a duty of 100% with thecyan, magenta, yellow and black ink compositions. Plain paper Xerox P(manufactured by Fuji Xerox) and Xerox 4024 (manufactured by Xerox) wereused as recording media. The resulting printed material was allowed tostand under normal conditions for an hour, and then the lightness (Lvalue) of patch portions of the pattern was measured with a GRETAGdensitometer (manufactured by GretagMacbeth. The results are shown inTables 3 to 5.

Test 3: Color Developability (Optical Density, OD Value)

The OD value of the patch pattern (solid image) printed at a duty of100% in the same manner as in Test 2 was measured five times (fivepoints) with GRETAG densitometer (manufactured by GretagMacbeth). Thearithmetic mean of the OD value was calculated for each ink composition.The optical density was evaluated from the obtained average OD valueaccording to the following criteria:

A: 1.2≦OD

B: 1.1≦OD<1.2

C: OD<1.1

The results are shown in Tables 3 to 5.

Test 4: Color Bleeding 1 (Sharpness of Characters 1)

A background including cyan, magenta and yellow colors and then blackcharacters on the background were printed at a duty of 100% with thesame ink jet printer as used in the above tests. Plain paper Xerox P(manufactured by Fuji Xerox) and Xerox 4024 (manufactured by Xerox) wereused as recording media. The printed materials were allowed to standunder normal conditions for an hour, and then the bleeding of the blackcharacters into the background was visually observed for each printedmaterial and evaluated according to the following criteria:

A: No bleeding was observed.

B: Slight bleeding was observed, but was within an acceptable range.

C: Bleeding outside an acceptable range was observed.

The results are shown in Tables 3 to 5.

Test 5: Color Bleeding 2 (Sharpness of Characters 2)

Using the same ink jet printer as used in the above tests, a backgroundwas printed at a duty of 100% with an ink composition (single color,cyan or magenta) disposed at the upstream side, and then characters wereprinted on the background with an ink composition (single color, magentaor cyan) disposed at the downstream side. Plain paper Xerox P(manufactured by Fuji Xerox) and Xerox 4024 (manufactured by Xerox) wereused as recording media. The printed materials were allowed to standunder normal conditions for an hour, and then the bleeding of thecharacters into the background was visually observed for each printedmaterial and evaluated according to the following criteria:

A: No bleeding was observed.

B: Slight bleeding was observed, but within an acceptable range.

C: Bleeding outside an acceptable range was observed.

The results are shown in Tables 3 to 5.

Test 6: Feathering (Sharpness of Characters 3)

Black characters were printed on the white background of a recordingmedium, that is, directly on the recording medium, using the same inkjet printer as used above. Plain paper Xerox P (manufactured by FujiXerox) and Xerox 4024 (manufactured by Xerox) were used as recordingmedia. The printed materials were allowed to stand under normalconditions for an hour, and then the bleeding of the characters into thewhite background was visually observed for each printed material andevaluated according to the following criteria. “Feathering” mentionedherein refers to a phenomenon of spreading ink from the edge of theprinted portion on paper or a recording medium in a feather-like manner.Since the pulp fiber is hydrophilic and porous, ink spreads along thelength of the fibers, and thus this phenomenon occurs.

A: No feathering was observed.

B: Slight feathering was observed, but was within an acceptable range.

C: Feathering outside an acceptable range was observed.

The results are shown in Tables 3 to 5.

TABLE 3 Example1 Ink composition Y1 M1 C1 K1 Yield value (mPa)  0.22 0.28  0.43  1.49 Lightness 88.9  53.1  45.0  23.0  OD value A A A ACharacter sharpness 1 — — — A Character sharpness 2 — B B — Charactersharpness 3 — — — A

TABLE 4 Example2 Ink composition Y2 C2 M1 K2 Yield value (mPa)  0.11 0.19  0.28  1.88 Lightness 87.7  55.6  53.1  22.8  OD value A A A ACharacter sharpness 1 — — — A Character sharpness 2 — B B — Charactersharpness 3 — — — A

TABLE 5 Comparative Ink composition Y3 M1 C2 K3 Example Yield value(mPa)  0.31  0.28  0.19  0.05 Lightness 88.9  53.1  55.6  22.1  OD valueA A A A Character sharpness 1 — — — C Character sharpness 2 — C C —Character sharpness 3 — — — B

What is claimed is:
 1. An ink jet recording method comprising: ejectingat least two color ink compositions onto a recording medium so as to bedeposited one on another; wherein the at least two color inkcompositions include a black ink composition having a yield value Y_(K),a cyan ink composition having a yield value Y_(C), a magenta inkcomposition having a yield value Y_(M), and a yellow ink compositionhaving a yield value Y_(Y), and the yield values satisfy therelationship: Y_(K)>Y_(C)>Y_(M)>Y_(Y); and wherein when one inkcomposition is deposited to form a first image and then another inkcomposition is deposited to form a second image on the first image, theink composition forming the second image has a higher yield value thanthe ink composition forming the first image.
 2. An ink jet recordingmethod comprising: ejecting at least two color ink compositions onto arecording medium so as to be deposited one on another; wherein the atleast two color ink compositions include a black ink composition havinga yield value Y_(K), a magenta ink composition having a yield valueY_(M), a cyan ink composition having a yield value Y_(C), and a yellowink composition having a yield value Y_(Y), and the yield values satisfythe relationship: Y_(K)>Y_(M)>Y_(C)>Y_(Y); and wherein when one inkcomposition is deposited to form a first image and then another inkcomposition is deposited to form a second image on the first image, theink composition forming the second image has a higher yield value thanthe ink composition forming the first image.